The present invention relates to a packaging container for the cold storage of liquid foods, for retaining superior flavour and aroma properties in the packed product, the container being of the type which is produced by fold forming and sealing of a planar, substantially rectangular or quadratic packaging container blank comprising side wall panels, top panels and bottom panels, for the formation of a tubular blank with a longitudinal sealing joint between the overlapping longitudinal incision edges of the packaging container blank, the tubular blank thereafter being given the desired bottom and top closures by fold forming and sealing of the bottom and top panels, respectively, of the packaging container blank, the packaging container blank being produced from a laminated packaging material comprising a core layer of paper or paperboard, an outer, aroma barrier layer of PET and a gas barrier layer disposed between the core layer and the aroma barrier layer. The present invention also relates to a method of producing such packaging containers.
Use has long been made within the packaging industry of packages of a single-use nature (so-called single-use disposable packages) for packing and transporting products such as liquid foods. A very large group of these single-use disposable packages is produced from laminated packaging material based on an interjacent core layer of paper or paperboard and outer laminate layers of some thermosealable plastic possessing superior liquid barrier properties, normally such as low density polyethylene (LDPE).
Depending on what food product is to be packed, i.e. its composition and storage sensitivity, its shelf-life, additional laminate layersxe2x80x94or laminate layers of other types than LDPExe2x80x94may be included in the laminate structure. Examples of such additional or other laminate layers may be material layers possessing superior gas barrier properties, such as an aluminium foil or a layer of polyamide or of copolymers of ethylene and vinyl alcohol. Certain food products, such as juice, moreover place more stringent requirements to the effect that the packaging material possess superior aroma barrier properties, i.e. prevent flavour deterioration as a result of non-polar flavour and aroma substances being absorbed from the packed product into the packaging material. At the same time as the package must afford the product the best possible product protection properties, production of such single-use packages must also be simple and rational in order to be economically viable.
Within the prior art technology, polyethylene terephthalate (PET) has often been proposed as a material possessing superior aroma barrier properties, suitable for the inside layer in a packaging container for direct contact with the packed product, as opposed to, for example, LDPE. PET possesses extremely good barrier properties against essential oils such as D-limonen and other non-polar flavour and nutrient substances in, for example, orange juice and is, therefore, a highly desirable material for this purpose. However, PET suffers from the major drawback in employment as the innermost laminate layer in a packaging container for direct contact with the packed product in that it is difficult to thermoseal at rational production speeds, in particular on sealing of the longitudinal joints in a packaging container produced from a sheet-shaped packaging laminate blank in which the longitudinal edges of the sheet-shaped blank overlap one another and are exposed such that the outside of the inner edge is sealed against the inside of the outer edge. In rational production of conventional packaging containers, such longitudinal joint sealing takes place at very high speeds, in that the sheet-shaped blanks in rapid sequence are advanced, reformed and longitudinally sealed by means of thermosealing into tubular packaging container blanks. The term xe2x80x9ctubularxe2x80x9d is hereafter taken to signify tubes of both circular and quadratic or rectangular cross section. For thermosealing of PET, it is necessary that the pressure from the sealing jaws is maintained during the heating process, at least up to approximately 165xc2x0 C., which takes roughly 0.5 sec. However, the available stay time during the sealing process on sealing of longitudinal joints, i.e. the time during which the pressure from the sealing jaws is maintained, is only approx. 0.01 sec., and thereby insufficient. On the other hand, sealing of the top and bottom of the same packaging containers takes place intermittently in connection with the product being filled into the container, which permits longer stay times in the sealing operation proper, and thereby makes for thermosealing by means of surface fusion between two PET layers.
Attempts have been made to overcome these difficulties in various manners, for example by employing a modified PET which facilitates thermosealing. From, for example, European Patent Application EP 0 237 235, it is known that glycol-modified PET, so-called PETG, may be thermosealed. However, a serious drawback inherent in this glycol-modified PET is that it results in a more brittle material layer with less flexibility and durability and is thus not as desirable in a packaging laminate as normal, amorphous, non glycol-modified PET. Moreover, nor can thermosealing take place using PETG at such high production speeds as are actually desirable.
One method of attempting to circumvent the difficulties of longitudinally sealing normal PET by means of thermosealing has been instead to seal the longitudinal lap joints by means of hot melt glue sealing, by applying a hot melt glue along one edge and then compressing it with the other edge in the lap joint between the two edges. However, it has not hitherto been possible to achieve rational sealing speeds using this technique. It has not hitherto been possible to use hot melt glue application at acceptable speeds, and serious problems have been encountered with large spillage of hot melt glue and with the fact that the application of the hot melt glue becomes uneven, with tacky outflows as a result. Another difficulty in the striving to produce a packaging container possessing superior aroma barrier properties is that such a hot melt glue sealed longitudinal joint is not durable in cold storage. Such a packaging container has thus not hitherto been capable of being stored for a lengthy period of time with good liquid-, gas-, and aroma barrier properties, because of the fact that the hot melt glue sealed longitudinal joint becomes untight.
In respect of liquid-, gas-, and aroma barrier properties, such incision edges of the sheet-shaped packaging blank which are freely exposed to the packed product create problems in that gas and liquid molecules, like non-polar flavour substances, are slowly absorbed in the packaging material through the thus freely exposed incision edges.
Another drawback is that configurationally stable, strong and durable packaging containers according to the prior art technology normally require larger quantities of the materials included in the package in order to achieve improved stability and improved barrier properties, despite lengthy cold storage, and thus command a higher price.
Hence, it has not hitherto been possible in the prior art technology, in a cost effective and rational manner to longitudinally joint seal and produce fold-formed packaging containers from sheet-shaped blanks of a paper-based packaging laminate with inside layers of PET which possess retained superior gas and aroma barrier properties also in the lengthy cold storage of liquid foods.
One object of the present invention is therefore to realise a novel packaging container of the type described by way of introduction without the attendant problems of the type intimately to the prior art technology.
A further object of the present invention is to realise a configurationally stable packaging container possessing superior gas and aroma barrier properties for packing and lengthy cold storage of liquid foods, from a sheet-shaped blank of a packaging laminate.
One particular object of the present invention is to realise a fold-formed packaging container which, in terms of material consumption and production process, is cost effective and which, also during lengthy cold storage of liquid foods, maintains good configurational stability and good gas and aroma barrier properties.
Yet a further object of the present invention is to realise a simple and rational method of producing a packaging container possessing superior lengthy cold storage properties according to the present invention.
The point of departure for attaining the desired aroma barrier properties in a packaging container for lengthy storage of liquid foods is thus a packaging laminate comprising an inside layer, i.e. that layer which is turned to face inwards in a packaging container produced from the packaging laminate, for direct contact with packed product, possessing superior aroma barrier properties, such as polyester, suitably PET, and preferably amorphous PET. Certain current process-facilitating additives employed in the prior art technology may be added to amorphous PET for facilitating extrusion and application of such a film or layer of PET.
Suitable core layers in packaging laminates for packaging containers according to the invention consist of paper or paperboard of suitable packaging quality.
To obtain superior gas barrier properties for protecting the packed product, such as, for example, the vitamin C content of orange juice, a separate layer is required of a material possessing superior gas barrier properties. Polymer gas barrier materials are today most desirable in the new development of packaging materials, since, from both the point of view of recycling and the environment, and from the point of view of costs, they are deemed to be preferable. Well-known polymer gas barrier materials are, for example, polyamide (PA) or copolymers of ethylene and vinyl alcohol (EVOH). The most preferred material alternative for the packaging container according to the pertinent invention is polyamide or a mixture of different polyamides because of their excellent gas barrier properties and inherent material rigidity. Mixtures of substantially polyamide and PET or EVOH are well-functioning gas barrier alternatives in a packaging container according to the present invention.
A further surprising advantage with a packaging laminate including an inner layer of PET and a gas barrier layer of substantially PA is that such aroma and gas barrier layers independently make considerable contributions to the total rigidity of the packaging laminate and that the rigidity of the core layer is, as a result, not as critical. By optimising the material quantity in the core layer and each respective barrier layer, a cost effective packaging laminate can be obtained given the qualitative advantages which are achieved at the same time. It has surprisingly proved that, when a PET layer with a grammage of approx. 18-30 g/m2 is combined with PA layer with a grammage of approx. 5-15 g/m2, it is possible, with retained overall rigidity, to employ a xe2x80x9cweakerxe2x80x9d and therefore cheaper paper for the core layer. By such means, the rigidity of the paper core layer may be reduced from 360 mN to between 280 and 340 mN. A preferred paper core layer according to the invention thus has a rigidity of approx. 280-340 mN, preferably approx. 290-330 mN.
Preferably, the PET layer has a grammage of approx. 18-25, most preferably approx. 20 g/m2 for optimum aroma barrier properties, which, in measurement tests, has been defined as an at least 90% retention of D-limonen in the packed product. The grammage should preferably not be less than 20 g/m2, but the aroma barrier layer functions well even at approx. 18 g/m2.
The gas barrier layer is thus constituted preferably by PA and is then applied preferably in a quantity of approx. 8-12 g/m2, most preferably approx. 10 g/m2, which gives optimum gas barrier and rigidity properties.
The gas barrier layer may be laminated to the core layer by means of an interjacent layer of LDPE and/or an adhesive layer of an adhesive polymer, such as, for example, an acid-modified polyethylene. For example, adhesives of the type maleic acid anhydride-modified polyethylene function very well according to the invention. Preferably, the gas barrier layer is laminated to the core layer by means of a lamination layer of LDPE most proximal the core layer and an adhesive layer between the LDPE layer and the gas barrier layer, for obtaining optimum adhesion and laminate strength. However, it is also conceivable, depending on the quality and properties of the polyamide layer, that this is applied direct on the core layer by means of extrusion.
Preferably, the inside/aroma barrier layer is laminated to the gas barrier layer with the aid of an interjacent adhesive layer of the same type of acid-modified polyethylene.
On the outside of the packaging laminate, i.e. that side which forms the outside of a packaging container produced from the packaging laminate, a layer of a thermosealable polymerxe2x80x94preferably LDPExe2x80x94is suitably applied.
With a view to achieving the requisite superior gas and aroma barrier properties in lengthy cold storage of the packaging container according to the present invention, it is important to protect the incision edges of the sheet-shaped packaging blank from contact with the packed product in the packaging container. This may be put into effect in different, per se known manners, for example by applying separate protective strips over the incision edges. Since the problem with difficultly sealable PET layers remains unsolved, the covering of the incision edges with separate strips is not a good solution.
By, instead, splitting and partly removing, by so-called xe2x80x9cskivingxe2x80x9d, the longitudinal edge of the sheet-shaped packaging laminate blank which is on the inside of a container produced from the packaging blank, i.e. the inner edge, for the formation of a projecting strip of half of the thickness of the packaging laminate including the inside layer of PET and folding back and sealing the projecting strip against the outside of the packaging laminate, the incision edge is concealed behind the outer edge (see FIG. 2) and, thus, does not come into contact with the packed product. The skived and double folded strip is sealed against the outer overlapping edge""s PET inside, which avoids the situation that incision edges and other material than PET come into direct contact with the packed product in the longitudinal joint region of the inside of the packaging container.
The skived and double folded strip including the inside layer of PET is sealed to the outside of the inner edge partly by means of thermosealing along a narrow region in which the outermost longitudinal edge of the strip meets the non-skived outside layer of LDPE of the packaging laminate, and partly in that the hot melt glue applied for the longitudinal joint sealing flows out to the outermost edge of the strip and seals this against the unsplit packaging laminate in the inner edge.
The top and bottom portions of the packaging container are also fold formed in such a manner that exposure of incision edges to the packed product is precluded. One example of possible top and bottom fold formation techniques according to the invention is represented by a gable top package of the xe2x80x9cTetra Rexxe2x80x9d(copyright) type. In order to avoid incision edge exposure in the fold formed bottom in such a packaging container, one of the bottom panels may, in a per se known manner, be provided with a small projection which may be folded outwards towards the outside of the packaging container and sealed in beneath the outermost bottom panel in order to turn the incision edge outwards away from the inside of the packaging container.
By, for example, such methods of skiving and panel folding, a packaging container is obtained in which all contact surfaces vis-à-vis the packed product on the inside of the filled and sealed container consist of the outer layer of PET.
The above-described packaging laminate, intended for a packaging container according to the present invention, is preferably produced by multilayer co-extrusion of two or more of the gas barrier layer, the adhesive layers and the aroma barrier layer on the one side of the core layer. It is naturally also possible to prefabricate a film of the inside layer of the packaging laminate which is laminated to a core layer or an LDPE-coated core layer. According to one preferred manner of manufacture, all layers are co-extruded on the inside of the packaging laminate by means of a five-layer extrusion nozzle on the core layer. According to another preferred manner of manufacture, the gas barrier layer, the adhesive layers and the aroma barrier layer may be co-extruded as four layers on a core layer coated with LDPE. According to an alternative, preferred manner of manufacture, it is possible to apply the gas barrier layer, the polyester layer and an interjacent adhesive layer by means of co-extrusion direct on the core layer.
The skived and double folded edge is sealed against the overlapping outer edge""s inside with the aid of a hot melt glue possessing superior application, gluing and tightness properties, these properties being thoroughly retained even after lengthy cold storage, i.e. storage at refrigeration temperatures of approx. 4-8xc2x0 C. for at least six weeks. A suitable hot melt glue for the present invention must provide good adhesion to different substrates and good resistance to cold, i.e. be flexible even at low temperatures. In the practical application according to the invention, it is thus desirable that the hot melt glue does not become brittle and lose its tacky and adhesion capabilities at cold storage temperatures, i.e. at approx. 4-8xc2x0 C., even after a time as long as at least six weeks. At the same time, it is necessary that the glue has a high so-called tacking point and high viscosity in order to give sufficiently good adhesion properties to PET which in turn means that the molten glue must be applied at a relatively high temperature. Furthermore, a high tacking or softening point ensures that the longitudinal hot melt sealing joint will not be negatively influenced by the high sealing temperature of the subsequent intermittent transverse, or top and bottom, heat sealing operation, by re-melting or re-softening of the hot melt in the joint. A well-functioning example of such a hot melt glue is based on a copolymer of ethylene and vinyl acetate (EVA) in a composition with a tackifying rosin ester resin, the hot melt glue having a preferred thermal viscosity (xe2x80x9cThermoselxe2x80x9d) of approx. 30000-40000 cp, more preferably 36500 cp at 190xc2x0 C. (375 F) (27/5.0) (alternatively less preferably, a Thermosel viscosity of approx 60000 cp at 177 C (350 F) (27/2.5) or approx. 25000 cp at 204 C (400 F) (27/5.0)), and a plasticising or softening temperature at approx. 80-100xc2x0 C., preferably approx. 90-100xc2x0 C. Preferably, for sake of optimal properties at cooling temperature, the hot melt composition comprises 50-70% by weight of EVA, 20-40% by weight of rosin ester resin and 5-15% by weight of a synthetic plasticising polymer, which has a similar type of plasticising effect as mineral oil, and has a density of about 0.96-0.98, more preferably about 0.97-0.98 g/cm3.